Projection blackboard

ABSTRACT

A projection blackboard having both of an excellent blackboard function and an excellent screen function, and a process for producing the same. The surface enamel layer that is an outermost layer of the enamel layers is formed by using a enameling upper glaze wherein at least titanium oxide coated particles, or a titanium oxide glaze together therewith is blended into a. transparent mat glaze, so as to render the layer an enamel layer having a predetermined color tone, a surface characteristic that the Rz is from 5 to 25 μm and a reflective characteristic that the peak gain is 0.28 or more, or having these properties and a glossiness Gs(75°)of 1 to 30%. This makes it possible to produce a blackboard excellent in both of blackboard function and screen function. The titanium oxide coated particles are preferably particles wherein surfaces of mica particles are coated with titanium oxide.

TECHNICAL FIELD

The present invention relates to a blackboard (also referred as chalkboard) having an enamel layer on its surface, in particular, an enamelblackboard having both of an excellent blackboard function and anexcellent screen function. The wording “excellent blackboard function”referred to in the present invention means that a blackboard has afunction that characters, pictures or the like are easily written/drawnthereon with a chalk and are easily watched, and the written/drawncharacters, pictures or the like thereon are easily erased. The“excellent screen function” means that a blackboard has a function thatwhen images of characters or the like are projected thereon through aprojector, an OHP or the like, the images of the characters or the likecan be clearly projected.

BACKGROUND ART

Hitherto, blackboards (chalk boards) have a function that characters,pictures or the like are erasably written/drawn on the surface thereofwith a chalk, and have been used to assist various explanations,lectures, speeches or the like. Thus, in conventional blackboards (chalkboards) as prescribed in JIS S 6007, the surface thereof hasappropriately roughness in order that characters, pictures or the likecan easily be written/drawn with a chalk or can easily be erased, andfurther dark color material has been used for the surface thereof inorder that the written/drawn characters, pictures or the like can easilybe watched.

In recent years, however, it has been become general that variousprojectors such as lantern slides or OHPs are used to project materialsprepared in advance onto a screen and then an explanation is made whileeither one of the projected images is pointed with a painter or thelike. In the case of making an explanation or the like using variousprojectors, it is necessary to prepare both of a projecting screen and ablackboard, on which characters or the like can be written/drawn with achalk and can be erased to make a supplementary explanation and thus,considerably much time is required for the arrangement thereof and tiresettlement thereof. The reason therefor is as follows: when images areprojected to a conventional blackboard with any one of variousprojectors, the quality of the projected images are dark and unclear,thus, no conventional blackboard has been used as a screen.

On the other hand, in reflection screens, a main object of which is thatimages through various projectors is projected thereon, material inwhite color or a highly bright color is used therefore in order toimprove the reflective property. Accordingly, when writing or drawing ismade onto such a screen with a chalk (which chalk), a low contrast isgiven. Thus, the recognizable property of characters, pictures or thelike decreases. Ordinary screens such as beads or white mats have a lowsurface hardness; thus, when writing or drawing is made thereon with achalk (white chalk) and the resultant is erased, a problem that thescreens are worn away is caused. Therefore, as disclosed in JapanesePatent Application Laid-Open (JP-A) No. 02-153080 and Japanese PatentApplication Laid-Open (JP-A) No. 04-229299, screens are not used aschalk boards although the screens are used as marker boards, on whichwriting or drawing with a marker and erasing are made.

Against such problems, for example, Japanese Utility Model ApplicationLaid-Open (JP-U) No. 57-183190 suggests an enamel blackboard which isalso used as a projecting screen, having a rough-surface enamel layer onits surface which informed on a steel sheet using a glaze containing ablue dye and have a brightness of 4 to 8 on a steel sheet.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, the blackboard produced by the technique described in JapaneseUtility Model Application Laid-Open (JP-U) No. 57-183190, has lowreflective luminance and shows a tendency to give an unclear image. Inparticular, in the case of using a projector giving a low luminance, aclear image can hardly be obtained on that black board. In such a way,there remain problems that the blackboard cannot satisfy recentproperties required for a screen sufficiently. An object of the presentinvention is to solve the above-stated problems in the prior artfavorably and provide a projection blackboard having both of anexcellent blackboard function and an excellent screen function, on whichcharacters, pictures or the like are easily written/drawn with a chalkand are easily watched so as to give an excellentchalk-character-recognizable property, on which characters, pictures orthe like written/drawn with a chalk can easily be erased to give anexcellent chalk-character-writable/drawable/erasable property besides anexcellent chalk-character-recognizable property, and on which projectedimages can be clearly screened to give an excellentprojected-image-recognizable property; and a producing process thereof.The projection blackboard which is a target of the present invention isan enamel blackboard having one or more enamel layers on an outersurface of a metallic substrate.

Means for Solving the Problems

In order to attain the object stated above, the inventors have madeeager investigations on factors producing effects onto thechalk-character-recognizable property, thechalk-writable/drawable/erasable property, and theprojected-image-recognizable property of an enamel blackboard. As aresult, the inventors have found out that it is important to prepare asurface enamel layer of a blackboard in such a manner that: the surfaceenamel layer has a brightness in a predetermined range and, a surfaceroughness in a predetermined range to keep certainlychalk-character-recognizable property,chalk-character-writable/drawable/erasable property which are equal toor more than those of conventional blackboards; and further the layerhas such a reflective property that the layer has a peak gain not lowerthan a predetermined value in order to improve theprojected-image-recognizable property.

It has been newly found out that such a surface enamel layer can beproduced by using a glaze to be used a glaze wherein at least anappropriate amount of titanium oxide coated particles, or a titaniumoxide glaze together therewith is blended into a transparent mat glaze,which is an ordinary chalk board glaze, and by adjusting the firingconditions into an appropriate range. It has been found out that thismanner makes it possible to cause a blackboard to have both of anexcellent blackboard function and an excellent screen function.

On the basis of the finding stated above, the present invention has beenmade by the addition of a further investigation thereto.

Accordingly, the subject matters of the present invention are asfollows:

(1) A projection blackboard, comprising one or more enamel layers on ametallic substrate, wherein a surface enamel layer that is an outermostlayer of said one or more enamel layers has a color tone that thebrightness V prescribed in JIS Z 8721-1993 is from 3.0 to 7.0, a surfacecharacteristic that the surface roughness Rz prescribed in JIS B0601-2001 is from 5 to 25 μm, and a reflective characteristic that thepeak gain is 0.28 or more.

(2) The projection blackboard according to item (1), wherein saidsurface enamel layer is an enamel layer formed by using of an enamelingupper glaze wherein at least titanium oxide coated particles, optionallya titanium oxide glaze together therewith, or a transparent gloss glazetogether with the two is blended into a transparent mat glaze.

(3) The projection blackboard according to item (1) or (2), wherein thesurface enamel layer has any one of black, green and brown colors.

(4) The projection blackboard according to any one of items (1) to (3),wherein the surface enamel layer has a 75-degree mirror plane glossinessGs (75°) of 1 to 30%, the Gs (75°) being prescribed in JIS Z 8741-1997.

(5) The projection blackboard according to any one of items (1) to (4),wherein the titanium oxide coated particles are particles whereinsurfaces of mica particles are coated with titanium oxide.

(6) A process for producing an projection blackboard, wherein: anenameling lower glaze is coated onto a surface of a metallic substrateand the resultant is fired to form an underlying enamel layer, or nounderlying enamel layer is formed; an enameling upper glaze in which atleast titanium oxide coated particles, a titanium oxide glaze togethertherewith, or a transparent gloss glaze together with the two is blendedinto a transparent mat glaze is then coated thereon; and the resultantis fired at 400 to 800° C. to form a surface enamel layer.

(7) The process for producing a projection blackboard according to item(6), wherein the metallic substrate is a cold-rolled steel sheet, analuminum plated steel sheet, or a Zn—Al alloy plated steel plate.

(8) The process for producing a projection blackboard according to item(6) or (7), wherein the titanium oxide coated particles are particleswherein surfaces of mica particles are coated with titanium oxide.

ADVANTAGEOUS EFFECTS OF THE INVENTION

According to the present invention, it is possible to produce, at lowcosts, a projection blackboard which is excellent inchalk-character-recognizable property, chalk-character-erasableproperty, and image-recognizable property, is easily handled and is alsoexcellent in constructability. Thus, industrially advantageous effectsare remarkably produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing examples of reflective characteristic curves.

FIG. 2 is a schematic view showing a method for measuring reflectiveluminance.

Description of Reference Numerals  1 incident light  7 reflected light 3s blackboard to be measured (sample) 10 light source 11 luminancemeter

BEST MODE FOR CARRYING OUT THE INVENTION

The projection blackboard according to the present invention is ablackboard having one or more enamel layers on a metallic substrate. Inthe present invention, the kind of the used metallic substrate is notparticularly limited. The substrate is preferably a cold-rolled steelsheet. It is preferred to plate the cold-rolled steel sheet used as themetallic substrate with Ni in order to improve the adhesive property ofthe metallic substrate to a glaze. Examples of the cold-rolled steelsheet include a low-carbon steel sheet and a stainless steel sheet. TheC content in the low-carbon steel sheet is preferably 0.0200% or less bymass, more preferably 0.0100% or less by mass, even more preferably0.0050% or less by mass.

The metallic substrate may be an aluminum plated steel sheet or a Zn—Alalloy plated steel sheet. Examples of the Zn—Al alloy plated steel sheetinclude a plated steel sheet having a plating layer consisted of 4 to70% by mass of Al and the balance made of Zn and inevitable impurities.It is needless to say that an additive for improving a property of theplated coat may be included into the plating layer.

The surface enamel layer that is an outermost layer out of the enamellayers formed on the metallic substrate has a color tone that thebrightness V prescribed in JIS Z 8721-1993 is from 3.0 to 7.0, a surfacecharacteristic that the surface roughness Rz prescribed in JIS B0601-2001 is from 5 to 25 μm, and a reflective characteristic that thepeak gain is 0.28 or more. The brightness V referred to in the presentinvention is a value measured in accordance with a color-measuringmethod prescribed in JIS Z 8722-2000 and represented by acolor-representing method prescribed in JIS Z 8721-1993.

The projection blackboard of the present invention is first required tohave a chalk-character-recognizable property andchalk-character-erasable properly which are equal to or more than thoseof conventional blackboards, so as to keep an excellent blackboardfunction.

For this purpose, in the present invention, the surface enamel layer isfirst rendered a surface enamel layer having a color tone that thebrightness V prescribed in JIS Z 8721-1993 is from 3.0 to 7.0 and thelayer is preferably in any one of black, green, brown and dark blue,more preferably in a dark color selected from black, green or brown. Ifthe brightness V falls down from the above-mentioned range, theblackboard becomes too dark so that the projected-image-recognizableproperty deteriorates when images are projected thereon. On the otherhand, if the brightness V rises up from the range, the blackboardbecomes too bright so that the chalk-character-recognizable propertydeteriorates. The color tone and the brightness of the enamel layer canbe adjusted by the kind of the pigments) to be added, or the addedamount thereof. In order to adjust the brightness V into the range of3.0 to 7.0, it is preferred to add, as one or more pigments, a metal Co,Cr or Ti, or oxides thereof alone or in combination of two or morethereof. The brightness V is preferably from 4.0 to 6.0.

In the projection blackboard of the present invention, the surfacecharacteristic of the surface enamel layer is adjusted in such a mannerthat the surface roughness Rz prescribed in JIS B 0601-2001 is turnedinto the range of 5 to 25 μm in order to keep certainly achalk-character-writable/drawable/erasable property, which is a propertythat characters, pictures or the like are easily written/drawn with achalk and are easily erased. The surface roughness is preferably from 10to 22 μm, more preferably from 14 to 18 μm.

If the surface roughness Rz of the surface enamel layer gets out of therange and the surface approximates a plain surface, a chalk slidesthereon so that the chalk-writable/drawable property deteriorates andfurther the projected-image-recognizable property deteriorates. On theother hand, if the roughness gets out of the above-mentioned range andthe irregularities become too large. the chalk-character-erasableproperty declines and further the projected-image-recognizable propertydeclines. As will be described later, the surface property of theabove-mentioned surface enamel layer can be attained by setting acombination of the composition of the glaze to be coated on the metallicsubstrate and the firing conditions after the glaze is coated into anappropriate range.

In the projection blackboard of the invention, the surface enamel layerhas the above-mentioned color tone and surface characteristic tomaintain an excellent blackboard function, and further has a reflectivecharacteristic that the peak gain is 0.28 or more, and preferably thehalf gain is 15 or more.

As shown in, for example, FIG. 2, the reflective characteristic of asheet to be measured is obtained from a reflective characteristic curve(see FIG. 1) gained as follows: incident light 1 is radiated from alight source 10 perpendicularly to the center of a plate 3 s to bemeasured; and a luminance meter 11 is used to measure the intensity ofreflected light 2 (reflective luminance) at each of points shiftedright-handedly and left-handedly in the range of 75°, respectively, fromthe central point at intervals of 5°on the arc of a single circle. The“peak gain” referred to in the present invention means the ratio of thereflective luminance of a blackboard to be measured which is measuredwhen light is radiated onto the blackboard to that of a standardwhiteboard (perfect diffusion plate: a purely white plate, on whichbarium sulfate is baked) measured under the same conditions. In thiscase, the reflective luminance at a viewing angle of 5° is usually used.The “half gain” referred to in the present invention means the viewingangle width (degree(s)) when the reflective luminance becomes ½ of thatat the peak in the reflective characteristic curve.

If the peak gain is less than 0.28, projected images are dark so thatthe projected-image-recognizable property becomes poor. As a result, theblackboard cannot satisfy requirements for a projection blackboard.Thus, the lower limit of the peak gain is set to 0.28. The peak gain ispreferably from 0.5 to 1.0. In order to obtain a wide viewing angle, thehalf gain is preferably set to 15 or more. For this reason, in thepresent invention, the surface enamel layer is rendered a surface enamellayer having a reflective characteristic of which has a peak gain of0.28 or more and preferably a half gain of 15 or more.

As will be described later, the above-mentioned reflectivecharacteristic of the surface enamel layer can be attained by: adding,to a glaze to be coated onto the metallic substrate, an appropriateamount of titanium oxide coated particles, preferably mica particlescoated with titanium oxide, having an appropriate particle diameterdistribution; and adjusting the firing temperature into an appropriaterange.

The surface enamel layer having the above-mentioned characteristicspreferably has such a surface gloss that the 75-degree mirror planeglossiness Gs (75°) is from 1 to 30%, the Gs (75°) being prescribed inJIS Z 8741-1997. If the Gs (75°) is less than 1%, thechalk-character-erasable property deteriorates and further thereflective luminance becomes small so that projected images get unclear.On the other hand, if the Gs (75°) is more than 30%, thechalk-character-writable/drawable property deteriorates and furtherhalation is easily caused to make the viewing angle narrow. The Gs (75°)is more preferably from 15 to 25%.

The surface enamel layer contains titanium oxide coated particles. Thecontaining of the titanium oxide coated particles into the surfaceenamel layer causes an improvement in the clearness, and causes theformation of irregularities on the surface to improve the diffusionreflectivity. Thus, halation is restrained so as to improve thereflective characteristic. In the present invention, the titanium oxidecoated particles are contained in the surface enamel layer preferably inan amount of 2 to 40% by mass of the whole of the surface enamel layer,more preferably in an amount of 3 to 20% by mass thereof. If the amountof the titanium oxide coated particles is less than 2% by mass, thereflective characteristic improving effect is small. If the amount ofthe titanium oxide coated particles is more than 40% by mass, thesurface roughness Rz becomes excessively large so that thechalk-character-erasable property deteriorates. Additionally, thebrightness becomes too large so that the chalk-character-recognizableproperty decreases.

The titanium oxide coated particles preferably have an average particlediameter of 100 μm or less, and more preferably have that of 5 to 80 μm.If the average particle diameter of the titanium oxide coated particlesis less than 5 μm, the diffusion reflectivity falls. On the other hand,if the average particle diameter of the titanium oxide coated particlesis more than 100 μm, the surface roughness of the surface enamel layerbecomes excessively large so that the chalk-character-erasable propertydecreases. The average particle diameter of the particles referred toherein means the 50% cumulated particle diameter measured by use of alaser diffraction type particle size distribution measuring device. Thetitanium oxide coated particles are preferably particles whereinsurfaces of mica particles are coated with titanium oxide. The micaparticles are preferably thin-plate-form mica particles.

The surface enamel layer having the above-mentioned properties arepreferably an enamel layer formed by using an enameling upper glazeobtained by blending, into a transparent mat glaze, at least titaniumoxide coated particles, optionally a titanium oxide glaze togethertherewith, or a transparent gloss glaze together with the two. Thethickness of the surface enamel layer is preferably 200 μm or less fromthe viewpoint of the peel property thereof. The thickness is morepreferably 150 μm or less.

The surface enamel layer formed by using the enameling upper glazestated above is preferably a enamel layer containing: TiO₂, which istotal amount of titanium oxide including titanium oxide in the titaniumoxide coated particles: 0.5 to 15% by mass, more preferably 0.5 to lessthan 10% by mass, more preferably 2 to 5% by mass; SiO₂: 15 to 45%;Al₂O₃: 5 to 30%.; the total of one or more selected from Na₂O, K₂O andLi₂O: 2 to 20% by mass; and B₂O₃: 2 to 20% by mass. In the presentinvention, it is needless to say that the enamel layer of the blackboardcontains one or more pigments selected from black pigments, greenpigments, and yellow pigments in a total amount of 20% or less by massin order to give a desired color.

The following will describe a preferred process for producing theprojection blackboard of the present invention.

The projection blackboard of the invention is preferably produced by aproducing process wherein: an enameling lower glaze is coated onto asurface of a metallic substrate and the resultant is fired to form anunderlying enamel layer, or no underlying enamel layer is formed; anenameling upper glaze in which at least titanium oxide coated particles,optionally a titanium oxide glaze together therewith, or a transparentgloss glaze together with the two is blended into a transparent matglaze is then coated: thereon; and the resultant is fired at 400 to 850°C. to form a surface enamel layer. The metallic substrate is preferablya cold-rolled steel sheet, an aluminum plated steel sheet, or a Zn—Alplated steel sheet.

When a cold-rolled steel sheet is used as the metallic substrate, it ispreferred to plate the cold-rolled steel sheet with Ni, coat anenameling lower glaze on a surface of the cold-rolled steel sheet (themetallic substrate), and fire the resultant to form an underlying enamellayer. The enameling lower glaze is not particularly limited, and ispreferably any enameling lower glaze that is used in conventional enamelblackboards. For the coating of the enameling lower glaze, any knowncoating manner may be used,

example thereof including spray and a roll coater.

In particular, the enameling lower glaze is preferably, for example, aSiO₂ based glaze comprising SiO₂: 20-80%, and the balance composed ofTiO₂: 0-15%, ZrO₂: 0-20%, B₂O₃: 0-25%, Al₂O₃: 0-25%, Na₂O: 0-20%, Li₂O:0-20%, K₂O; 0-20%, PbO: 0-40%, ZnO: 0-50%, BaO: 0-15%, CaO: 0-15%, CaF₂:0-10%, CoO: 0-20%, NiO; 0-20%, MnO: 0-20%, and so on, each “%”representing % by mass relative to the basis of the whole of solidcontents therein.

An enameling upper glaze is coated on the surface of the resultantunderlying enamel layer, and subsequently the resultant is fired at 600to 850° C. to form a surface enamel layer. For the coating of theenameling upper glaze, any known coating manner may be used, examplethereof including spray and a roll coater.

The enameling upper glaze is preferably a glaze wherein an appropriateamount of titanium oxide coated particles are added to a transparent matglaze (frit) or to a glaze wherein 100 to 20 parts by weight of atitanium oxide glaze (frit) or 20 parts or less by weight of atransparent gloss glaze (frit) together therewith is added to 100 partsby weight of a transparent mat glaze (frit). The added amount of thetitanium oxide coated particles is preferably from 1 to 25 parts byweight for 100 parts by weight (a value converted to weight after thefiring) of the transparent mat glaze or the mixed glaze (slip).

The enameling upper glaze is obtained by adding/mixing an appropriateamount of powder of titanium oxide coated particles to/with a slipobtained by grinding a mixture of: a frit of a transparent mat glaze, ora transparent mat glaze, a titanium oxide glaze, or further a frit of atransparent gloss glaze; optional various additives and one or moreoptional pigments; and water. It is necessary to add any one of blackpigment, green pigment, and yellow pigment or a combination of two ormore thereof, as the pigments), in accordance with a desired color toneof the blackboard. The total added amount of the pigment(s) ispreferably 20 parts or less by weight for 100 parts by weight (a valueconverted to weight, after the firing) of the slip. If the added amountis more than 20 parts by weight so that the amount of the pigment(s)becomes large, the fluidity of the slip decreases.

The transparent mat glaze (frit) is preferably a glaze comprising SiO₂:30-45%, Al₂O₃: 20-35%, B₂O₃: 5-15%, K₂O: 5-15%, Na₂O: 10-25%, and thebalance composed of BaO: 0-15%, CaO; 0-15%, F₂; 0-10%, TiO₂: 0-10%, andso on, each “%”representing % by mass relative to the basis of the wholeof solid contents therein.

The titanium oxide glaze (frit) is preferably a glaze comprising 10 to30 % of TiO₂ and the balance composed of SiO₂: 0-80%, Al₂O₃: 0-20%,B₂O₃: 0-25%, Na₂O: 0-20%, K₂O: 0-20%, Li₂O: 0-20%, P₂O₅: 0-10%, ZrO₂:0-20%, BaO: 0-15%, CaO: 0-15%, MgO: 0-5%, PbO: 0-40%, ZnO: 0-50%, CaF₂:0-10%, and so on, each “%” representing % by mass relative to the basisof the whole of solid contents therein. Instead of titanium oxide, whichis a pigment of the titanium oxide glaze, one or more pigments out ofzirconium oxide, antimony oxide and zinc oxide may be used.

The transparent gloss glaze (frit) is preferably a glaze comprisingSiO₂: 40-65%, and the balance composed of B₂O₃: 5-20%, one or more outof Na₂O, K₂O and Li₂O: 5-40% in total, F₂: 0-10%, Al₂O₃: 0-10%, CaO:0-10%, TiO₂: 0-10%, ZnO: 0-10%, MgO: 0-5%, and so on, each “%”representing % by mass relative to the basis of the whole of solidcontents therein. Instead of titanium oxide, which is a pigment of thetitanium oxide glaze, one or more pigments out of zirconium oxide,antimony oxide and zinc oxide may be used.

If the firing temperature is lower than 600° C., the surface roughnessbecomes too rough so that the reflective characteristic deteriorates. Onthe other hand, if the temperature is higher than 850° C., the surfacebecomes too flat and smooth so that the chalk-character-recognizableproperty and the chalk-character-writable/drawable/erasable propertydecline.

In the case of using, as the metallic substrate, an aluminum platedsteel sheet or a Zn—Al alloy plated steel sheet, it is preferred to coatan enameling upper glaze onto a surface of the metallic substrate andfire the resultant at 400 to 600° C. to form a surface enamel layer.

In this case, it is necessary to use an enameling upper glaze having alow melting point to prevent the plating layer from being melted. It istherefore preferred to use an enameling upper glaze wherein titaniumoxide coated particles or a titanium oxide glaze together therewith isblended into a glaze wherein phosphoric acid is blended instead of theabove-mentioned transparent mat glaze. The phosphoric acid content bypercentage in the glaze into which phosphoric acid is blended is 40% ormore by mass and is preferably 80% or less by mass, more preferably 70%or less by mass.

The glaze (frit) into which phosphoric acid is blended is preferably aglaze comprising P₂O₅: 40-80%, and the balance composed of SiO₂: 0-40%,Al₂O₃; 0-50%, B₂O₃: 0-25%, Na₂O: 0-20%, K₂O: 0-20%, Li₂O:0-20%, TiO₂:0-30%, Sb₂O₃: 0-25%, ZnO: 0-20%, BaO: 0-15%, CaO: 0-15%, MgO: 0-5%, PbO:0-10%, SrO: 0-20%, and so on, each “%” representing % by mass relativeto the basis of the whole of solid contents therein.

In the case of using, as the metallic substrate, an aluminum platedsteel sheet or a Zn—Al alloy plated steel sheet, an enameling upperglaze having a low melting point is used to prevent the plating layerfrom being melted; thus, the adhesiveness is improved so that it is notnecessarily essential to coat the enameling lower glaze.

Moreover, in the case of using, as the metallic substrate, an aluminumplated steel sheet or a Zn—Al alloy plated steel sheet, the adhesivenessof the enamel layer decreases if the firing temperature is lower than400° C. On the other hand, if the temperature is higher than 600° C., aproblem that the plating layer is melted is caused.

EXAMPLES Example 1

A cold-rolled steel sheet (an extremely low carbon steel sheetcontaining C: 0.0050% by mass) was used as each metallic substrate. Anickel plating layer was formed on each surface of the cold-rolled steelsheet in a nickel sulfate bath. Next, an enameling lower glaze wascoated onto each surface of the cold-rolled steel sheet, which wasplated with the nickel, in a roll coater manner. Thereafter, theresultant was fired at 800° C. to form an underlying enamel layer 30 μmin thickness. The used enameling lower glaze was a glaze obtained bycharging, into a ball mill, a blend material wherein clay: 6 parts byweight, magnesium carbonate: 0.15 part by weight, borax: 0.4 part byweight, silica: 1 part by weight, and water: 70 parts by Weight wereadded to a SiO₂ based glaze (frit); 100 parts by weight, and thenmixing/grinding the components into a slip.

Next, an enameling upper glaze was coated onto one of the surfaces ofthe resultant underlying enamel layer in a spraying manner. Thereafter,the resultant was fired at 750° C. to form a surface enamel layer 70 μmin thickness. In such a way, sample blackboards were produced. Aboutsome of the blackboards, the tiring temperature was varied in the rangeof 700 to 780° C.

To 100 parts by weight of a transparent mat glaze (frit) having acomposition shown in Table 1 were added clay; 5.5 parts by weight,sodium nitrite: 0.15 part by weight, silica: 0.5 part by weight, andwater: 70 parts by weight, and further to the mixture were added TiO₂,Sb₂O₅ and NiO: 1 part by weight in total, Cr₂O₃: 8.2 parts by weight, Coand Cr: 10.0 parts by weight in total, and Fe₂O₃, CuO, CoO, MnO and soon: 0.1 part by weight in total as pigments. The components weremixed/grinded in a ball mill to yield a slip A, To 100 parts by weightof a titanium oxide glaze (frit) having a composition shown in Table 2were added clay: 5.5 parts by weight, potassium chloride: 0.2 part byweight, sodium aluminate: 0.3 part by weight, and water: 54 parts byweight, and the components were mixed/grinded to yield a slip B.Thereafter, to the slip B: 100 parts by weight were added powder oftitanium oxide coated particle: 20 parts by weight (a value converted toweight after the firing) together with water: 45 parts by weight. Thecomponents were mixed to prepare a slip C. The slip A and the slip Cwere blended with each other at a ratio of 3 to 1 to prepare eachenameling upper glaze which was used in the present invention. In someof the glazes, the mixing ratio between the slip A and the slip C wasvaried. The titanium oxide coated particle powder was made of particleswherein surfaces of thin-plate-form mica particle were coated withtitanium oxide, and had an average particle diameter of 20 μm and atitanium oxide coated area ratio of 30%. The average particle diameterwas the 50% cumulated particle diameter measured by use of a laserdiffraction type particle size distribution measuring device.

About a conventional example, to 100 parts by weight of a transparentmat glaze (frit) having the composition shown in Table 1 were addedclay: 5.5 parts by weight, sodium nitrite: 0.15 part by weight, silica:0.5 part by weight, and water; 70 parts by weight, and further to themixture were added TiO₂, Sb₂O₅ and NiO: 0.5 part by weight in total,Cr₂O₃: 4.1 parts by weight, Co and Cr: 5.0 parts by weight in total, andFe₂O₃, CuO, CoO, MnO and so on: 0.05 part by weight in total aspigments. The components were mixed/grinded in a ball mill to yield aslip D. The slip D was used as an enameling upper glaze, and firing at atiring temperature of 700° C. was carried out.

About the surface enamel layer of each of the resultant sampleblackboards, the composition, the surface roughness, the color tone, theglossiness and the reflective characteristic were measured.

(1) Surface Roughness

In the measurement of the surface roughness, a contact type surfaceroughness meter was used to obtain the arithmetic average roughness Raand the maximum height Rz in accordance with JIS B 0601-2001.

(2) Color Tone

The color tone was measured in accordance with JIS Z 8722-2000, and thecolor tone of each of the sample blackboards was represented inaccordance with JIS Z 8721-1993.

(3) Glossiness

About the glossiness, the 75-degree mirror plane glossiness Gs (75°) wasobtained in accordance with JIS Z 8741-1997.

(4) Reflective Characteristic

As shown in FIG. 2. about the reflective characteristic, incident light1 was radiated from a light source 10 perpendicularly to the center of aplate 3 s to be measured, and a luminance meter 11 was used to measurethe intensity of reflected light 2 (reflective luminance) at each ofpoints shifted right-handedly and left-handedly in the range of 75°,respectively, from the central point at intervals of 5° on the arc of asingle circle. The peak gain and the half gain were then obtained. Aboutthe peak gain, the following was defined as the peak gain: the ratio ofthe reflective luminance of the sample blackboard which was measuredwhen light was radiated onto the blackboard to that of a standardwhiteboard (perfect diffusion plate: a purely white plate, on whichbarium sulfate was baked) measured under the same conditions. About thereflective luminance, the reflective luminance at a viewing angle of 5°was used.

Examples of the reflective characteristics of the blackboards of thepresent invention (the present invention articles) and the conventionalblackboard (the conventional article) are shown in FIG. 1. Along thevertical axis, the reflective luminance of the measured blackboards isrepresented by the percentage when the reflective luminance of thestandard luminance (the peak) is set to 100%.

(3) Blackboard Function

Furthermore, characters were written with a chalk on each of theresultant sample blackboards in accordance with JIS B 6007, and then theeasiness of the adhesion of the chalk and the easiness of the peeling ofthe chalk were judged. About the easiness of the adhesion of the chalk,the following were judged: whether or not unevenness was present in thelines when the lines were watched from a position 1 m apart from theblackboard surface; and whether or not the lines were clear when thelines were watched from a position 8 m apart therefrom. A case where nounevenness was present in the lines and further the lines were clear isrepresented by, a case where the clearness decreased slightly isrepresented by o, and a case where unevenness was present in the linesor the lines were not clear is represented by X. In such a way, thechalk-character-recognizable property was evaluated. About the easinessof the peeling of the chalk, the characters written on the blackboardwas rubbed with an eraser (made of felt), and then the blackboard waswatched at a position 1 m apart from the blackboard. A case where atrace of the handwriting and erasion unevenness were not present isrepresented by, a case where traces of the handwriting remained slightlyis represented by o, and a case other than the two cases is representedby X. In this way, the chalk-character-erasable property was evaluated.

The results are shown in Tables 3 and 4.

TABLE 1 Composition (% by mass) Na₂O + SiO₂ Al₂O₃ K₂O + Li₂O B₂O₃ TiO₂BaO CaO F₂ Balance Transparent 37 27 20 8 2 1 1 1 3 mat glaze

TABLE 2 Composition (% by mass) Na₂O + K₂O + SiO₂ Al₂O₃ Li₂O B₂O₃ TiO₂P₂O₅ F₂ Balance Titanium 35 3 18 13 22 2.5 4 2.5 oxide glaze

TABLE 3 Composition (% by mass) Na₂O + Color tone Titanium K₂O +adjusting oxide coated No. SiO₂ Al₂O₃ Li₂O B₂O₃ TiO₂ F₂ CaO BaO Balancepigment particles Surface a 31 15 15 8 12 2 0.4 0.4 16.2 8.2 8.4 enamelb 31 18 15 7 9 2 0.5 0.5 17.0 10.6 5.6 layer c 31 19 16 7 7 1 0.6 0.617.8 11.9 4.2 d 31 20 16 7 6 1 0.6 0.6 17.8 12.6 3.3 e 36 25 17 7 0.3 10.9 0.9 12.9 8.3 —

TABLE 4 Color tone Surface Chalk- Chalk- Reflective Black- SurfaceFiring C roughness Glossiness character- character- characteristic boardenamel temperature V (Bright- (Chromatic- Ra Rz Gs recognizable erasablePeak Half No. layer No. □ H (Hue) ness) ness) μm μm (75 D) propertyproperty gain gain Notes 1 a 750 7.1 G 6.1 2.9 2.3 16.2 18 ∘ □ 0.74316.2 The present invention example 2 b 750 7.0 G 5.3 3.3 2.2 15.5 15 □ □0.414 21.8 The present invention example 3 c 750 6.2 G 5.0 3.4 1.9 12.014 □ □ 0.368 22.0 The present invention example 4 d 750 5.5 G 4.6 3.41.9 12.7 14 □ □ 0.313 23.1 The present invention example 5 c 730 6.2 G5.2 8.2 1.8 24.0 5.8 □ ∘ 0.285 37.9 The present invention example 6 c740 8.3 G 5.1 2.9 2.1 19.1 9.2 □ □ 0.381 29.0 The present inventionexample 7 c 760 4.5 G 5.2 1.9 1.5 15.6 17 □ □ 0.543 21.1 The presentinvention example 8 c 780 7.5 G 5.6 1.7 1.4 14.9 23 □ □ 0.768 17.2 Thepresent invention example 9 e 700 2.7 G 3.4 1.9 1.3 18.5 15 □ □ 0.22022.8 Conventional example

The present invention examples each maintained achalk-character-recognizable property and a chalk-character-erasableproperty equivalent to those of the conventional blackboard to keep anexcellent blackboard function, and further gave a peak gain of 0.28 ormore to exhibit an excellent reflective characteristic. Thus, whenimages are projected thereon, the blackboards are blackboards excellentin projected-image-recognizable property and suitable forimage-projection. The comparative example outside the scope of thepresent invention had a lowered blackboard function ofchalk-character-recognizable property and chalk-character-erasableproperty, or had a poor reflective characteristic with declined theprojected-image-recognizable property thereof.

The invention claimed is:
 1. A projection blackboard, comprising: atleast one enamel layer on a metallic substrate, wherein an outermostsurface enamel layer of the at least one enamel layer is positioned asan outermost layer of the projection blackboard, said outermost surfaceenamel layer being an enameling upper glaze comprising a blend ofparticles coated with titanium dioxide and a transparent mat glaze, thesurface enamel layer having, i) a color tone with a brightness Vprescribed in JIS Z 8721-1993 from 3.0 to 7.0, ii) a surfacecharacteristic with a surface roughness Rz prescribed in JIS B0601-200from 5 to 25 μm, and iii) a reflective characteristic with apeak gain of 0.28 or more.
 2. The projection blackboard according toclaim 1, wherein said enameling upper glaze is further comprised of oneof i) a titanium dioxide glaze and ii) a titanium dioxide glaze and atransparent gloss glaze.
 3. The projection blackboard according to claim1, wherein said outermost surface enamel layer has any one of black,green and brown colors.
 4. The projection blackboard according to claim1, wherein said outermost surface enamel layer has a 75-degree mirrorplane glossiness Gs(75°) of 1 to 30%, the Gs (75°) being prescribed inJIS Z 8741-1997.
 5. The projection blackboard according to claim 1,wherein said particles coated with titanium dioxide are mica particlescoated with titanium dioxide.
 6. The projection blackboard according toclaim 1, wherein said metallic substrate is a cold-rolled steel sheet,an aluminum plated steel sheet, or a Zn—Al alloy plated steel plate. 7.The projection blackboard according to claim 1, wherein the particlescoated with titanium dioxide are present at 2 to 40% by mass in theoutermost surface enamel layer.
 8. The projection blackboard accordingto claim 1, wherein the particles coated with titanium dioxide arepresent at 3 to 20% by mass in the outermost surface enamel layer. 9.The projection blackboard according to claim 1, wherein the particlescoated with titanium dioxide have an average particle diameter of 100 μmor less.
 10. The projection blackboard according to claim 1, wherein theparticles coated with titanium dioxide have an average particle diameterof 5 to 80 μm or less.
 11. The projection blackboard according to claim1, wherein a thickness of the outermost surface enamel layer is 200 μmor less.
 12. The projection blackboard according to claim 1, wherein theoutermost surface enamel layer has a total amount of titanium dioxideincluding the titanium dioxide of the particles coated with titaniumdioxide is 0.5 to 15% by mass.
 13. The projection blackboard accordingto claim 1, wherein the outermost surface enamel layer has a totalamount of titanium dioxide including the titanium dioxide of theparticles coated with titanium dioxide is 0.5 to less than 10% by mass.14. The projection blackboard according to claim 1, wherein theoutermost surface enamel layer has any one of black, green and brownpigments in a total amount of 20% or less by mass.
 15. A projectionblackboard, comprising: at least one enamel layer on a metallicsubstrate, wherein an outermost surface enamel layer of the at least oneenamel layer, positioned as an outermost layer of the projectionblackboard, and said outermost surface enamel layer being an enamelingupper glaze comprising a blend of mica particles coated with titaniumdioxide and a transparent mat glaze, the surface enamel layer having, i)a color tone with a brightness V prescribed in JIS Z 8721-1993 from 3.0to 7.0, ii) a surface characteristic with a surface roughness Rzprescribed in JIS B 0601-2001 from 5 to 25 μm, and iii) a reflectivecharacteristic with a peak gain of 0.28 or more.
 16. A projectionblackboard, comprising: at least one enamel layer on a metallicsubstrate, wherein an outermost surface enamel layer of the at least oneenamel layer, positioned as an outermost layer of the projectionblackboard, and said outermost surface enamel layer being an enamelingupper glaze comprising a blend of 2to 40% by mass of mica particlescoated with titanium dioxide and a transparent mat glaze, the particlescoated with titanium dioxide having an average particle diameter of 100μm or less, a thickness of the outermost surface enamel layer being 200μm or less, the outermost surface enamel layer having a total amount oftitanium dioxide including the titanium dioxide of the particles coatedwith titanium dioxide being 0.5 to 15% by mass, the outermost surfaceenamel layer having, i) a color tone with a brightness V prescribed inJIS Z 8721-1993 from 3.0 to 7.0, ii) a surface characteristic with asurface roughness Rz prescribed in JIS B 0601-2001 from 5 to 25 μm, andiii) a reflective characteristic with a peak gain of 0.28 or more.